package tool;

//http://www.merriampark.com/comb.htm
//--------------------------------------
//Systematically generate combinations.
//--------------------------------------

import java.math.BigInteger;

public class CombinationGenerator {

private int[] a;
private int n;
private int r;
private BigInteger numLeft;
private BigInteger total;

//------------
// Constructor
//------------

public CombinationGenerator (int n, int r) {
  if (r > n) {
    throw new IllegalArgumentException ();
  }
  if (n < 1) {
    throw new IllegalArgumentException ();
  }
  this.n = n;
  this.r = r;
  a = new int[r];
  BigInteger nFact = getFactorial (n);
  BigInteger rFact = getFactorial (r);
  BigInteger nminusrFact = getFactorial (n - r);
  total = nFact.divide (rFact.multiply (nminusrFact));
  reset ();
}

//------
// Reset
//------

public void reset () {
  for (int i = 0; i < a.length; i++) {
    a[i] = i;
  }
  numLeft = new BigInteger (total.toString ());
}

//------------------------------------------------
// Return number of combinations not yet generated
//------------------------------------------------

public BigInteger getNumLeft () {
  return numLeft;
}

//-----------------------------
// Are there more combinations?
//-----------------------------

public boolean hasMore () {
  return numLeft.compareTo (BigInteger.ZERO) == 1;
}

//------------------------------------
// Return total number of combinations
//------------------------------------

public BigInteger getTotal () {
  return total;
}

//------------------
// Compute factorial
//------------------

private static BigInteger getFactorial (int n) {
  BigInteger fact = BigInteger.ONE;
  for (int i = n; i > 1; i--) {
    fact = fact.multiply (new BigInteger (Integer.toString (i)));
  }
  return fact;
}

//--------------------------------------------------------
// Generate next combination (algorithm from Rosen p. 286)
//--------------------------------------------------------

public int[] getNext () {

  if (numLeft.equals (total)) {
    numLeft = numLeft.subtract (BigInteger.ONE);
    return a;
  }

  int i = r - 1;
  while (a[i] == n - r + i) {
    i--;
  }
  a[i] = a[i] + 1;
  for (int j = i + 1; j < r; j++) {
    a[j] = a[i] + j - i;
  }

  numLeft = numLeft.subtract (BigInteger.ONE);
  return a;

}

public static void main(String args[]){
	int[] indices;
	CombinationGenerator x = new CombinationGenerator(5,3);
	while(x.hasMore()){
		indices = x.getNext();
		
		for(int i=0;i<indices.length;i++){
			System.out.print(indices[i]);
		}
		System.out.println("");
	}
}

}
